Image encoder, image encoding method, image decoder, image decoding method, and data recording medium

ABSTRACT

Apparatus for decoding, block by block, image block of N×N pixels, in coded image signal form obtained by successfully coding pixel values of plural peripheral pixels in the vicinity of a coding target pixel, the apparatus including means for replacing a pixel value of an undecoded pixel among three lines of nearby peripheral pixels with a pseudo pixel value obtained from a decoded peripheral pixel located closest to and on the same line as the uncoded pixel; means for generating a prediction pixel value for the target pixel from the value of the decoded pixel and the replacement pixel value of the undecoded pixel; means for (a) receiving the coded image signal, (b) performing a decoding process using the pixel values of at least one decoded pixel in each of a previously decoded block, the decoding target block, and a pseudo pixel value of an undecoded pixel, and (c) outputting a decoded image signal for each block; and means for combining decoded image signals corresponding to the blocks.

TECHNICAL FIELD

The present invention relates to image coding apparatuses and imagecoding methods, image decoding apparatuses and image decoding methods,and data recording media and, more particularly, to a coding process anda decoding process for performing recording or transmission of imagesignals with less bit number without degrading the image quality, and toa recording medium containing a program for realizing the coding processor the decoding process.

BACKGROUND ART

Conventional image coding processes are broadly divided into two ofcoding processes performed in block units, represented by MPEG2-basedcoding methods, and coding processes performed in pixel units, such asdifferential pulse code modulation (DPCM: Differential Pulse CodeModulation).

The coding process in block units is a method in which a single imagedisplay region is divided into plural blocks and a coding process for animage signal that is input (hereinafter referred to as an input imagesignal) is performed block by block. In this case, the single imagedisplay region corresponds to a single display screen in the MPEG2-basedcoding process, and in an MPEG4-based coding process, it corresponds toa display region having a shape and a size corresponding to each objecton a single display screen. Further, each of the above-mentioned blocksis a display region composed of a prescribed number of pixels within thesingle image display region, and a rectangle shape which is easilyprocessed is used as the shape of the block in many cases.

As described above, in the coding method in which a coding process foran input image signal is performed block by block, the coding processfor the input image signal corresponding to a single image displayregion is completed in each block. Therefore, there is the advantagethat, even though a transmission error occurs when a coded image signalobtained by performing the coding process for the input image signal istransmitted, influence of the error can be converged in each block.

However, the block-by-block coding method has the following drawbacks.

Since the coding process for the input image signal is completed in eachblock in the block-by-block coding method, it is difficult to use aninter-block pixel correlation, that is, a correlation of pixel valueswhich are present in different blocks, in the coding process.

Further, in a predictive coding method for an image signal, a pixelvalue of a coding target pixel being a target of coding (a coding targetpixel value) is predicted with reference to pixel values of plural codedpixels which have previously been coded (coded pixel values), and thecoding target pixel value is adaptively coded using the predicted pixelvalue. In this predictive coding method, however, when the codingprocess is performed block by block, the coded pixel values to bereferred to when coding the coding target pixel value are limited topixels within each block, so that the reference coded pixel values aresmall in number. Therefore, the accuracy of the predicted value of thecoding target pixel is reduced, and the coding efficiency is notincreased very much.

On the other hand, the coding method in pixel units is a method in whichan input image signal is coded pixel by pixel, and in this codingmethod, it is possible to change the coding process for the input imagesignal pixel by pixel. Therefore, when this coding method includes auniversal coding process, such as adaptive arithmetic coding in whichcode words are automatically updated pixel by pixel adaptively to thecharacteristics of the input image signal, an image signal having anycharacteristic can be coded with a significantly high coding efficiency.

However, since, on the decoding side, a coded image signal obtained bythe pixel-by-pixel coding method including the universal coding processis subjected to a decoding process in which code words are updated inthe same manner as on the coding side, when a transmission error occurswhen the coded image signal is transmitted, the state in which thedecoding process for the coded image signal cannot be carried outaccurately because of the influence of the transmission error on thedecoding side, continues long.

By the way, the block-by-block coding method and the pixel-by-pixelcoding method can be combined, and in a coding method in which thesecoding methods are combined (hereinafter referring to this coding methodas a combination coding method for explanation), code words can beadaptively changed for each pixel, and the influence of transmissionerror can be converged in each block, whereby a coding process with ahigh coding efficiency, such as adaptive arithmetic coding, can beperformed while suppressing the influence of transmission error.

A description is now given of this combination coding method.

FIG. 13(a) shows the state in which a single frame screen is dividedinto a plurality of rectangle blocks, and FIG. 13(b) shows arrangementof pixels in blocks, especially in a coding target block being a targetof coding and blocks in the vicinity of the coding target block.Needless to say, these pixels are arranged in matrix along horizontalscanning lines in the single frame screen.

In the figures, FG denotes a screen corresponding to a single frame, B1denotes a coded block in which a coding process for an image signal hasalready been performed, Bx denotes a coding target block being a targetof coding, and B0 denotes an uncoded block in which a coding process foran image signal has not been performed. When no distinction is madebetween blocks, blocks are denoted by B. BLu, BLs, BLh, and BLm denoteupper, lower, left, and right boundaries of the coding target block inthe single frame screen, respectively. A solid line circle shows a codedpixel whose pixel value has already been coded, and a dotted line circleshows an uncoded pixel whose pixel value has not been coded yet. Eachblock B is an image display region comprising 4×4 pixels, in the singleframe screen FG.

FIG. 14 shows positional relationships between a coding target pixel Pxto be coded and peripheral pixels P0˜P9 surrounding the coding targetpixel, and the pixel values of these peripheral pixels P0˜P9 arereferred to when the pixel value of the coding target pixel Px ispredicted, so that these pixels are called reference pixels hereinafter.

The reference pixels P8 and P9 are pixels which are positioned in thesame horizontal scanning line as the coding target pixel Px, and thereference pixels P9 and P8 are positioned one pixel and two pixelsbefore the coding target pixel Px, respectively. The positions of thereference pixels P5 and P1 in the horizontal direction on the singleframe screen FG correspond to the position of the coding target pixelPx, and the reference pixels P5 and P1 are positioned in a horizontalscanning line by one pixel and two pixels upper than the coding targetpixel Px, respectively. Further, the reference pixels P3, P4, P6, and P7are pixels which are positioned in the same horizontal scanning line asthe reference pixel P5, the reference pixels P4 and P3 are positionedone pixel and two pixels before the coding target pixel Px,respectively, and the reference pixels P6 and P7 are positioned onepixel and two pixels after the coding target pixel Px, respectively.Further, the reference pixels P0 and P2 are pixels which are positionedin the same horizontal scanning line as the reference pixel P1, thereference pixel P0 is positioned one pixel before the reference pixelP1, and the reference pixel P2 is positioned one pixel after thereference pixel P1.

In the combination coding method, initially, an image signalcorresponding to the single frame screen FG is divided correspondinglyto plural blocks B constituting the single frame screen as shown in FIG.13(a) and FIG. 13(b), and a coding process for the divided image signalis performed block by block.

This block-by-block coding process is completed by performing ahorizontal process in which pixel values of pixels are successivelycoded from the left to the right along each horizontal pixel line in ablock B, for all of the horizontal pixel lines in the block from theuppermost line to the lower most line.

In this coding process, as shown in FIG. 14, the pixel value of thecoding target pixel Px is adaptively predicted from the pixel values ofthe reference pixels P0˜P9 positioned in the vicinity of the codingtarget pixel, and code words used for the coding process of the codingtarget pixel are adaptively selected according to a predicted valueobtained by the prediction.

Therefore, in the combination coding method, the influence oftransmission error on the decoding side can be converged in each block,and the coding efficiency can be improved as compared with the simpleblock-by-block coding process.

FIG. 16(a), FIG. 16(b), and FIG. 17 are diagrams for explaining acombination decoding method corresponding to the above-mentionedcombination coding method, and in the figures, B′ denotes each block ina single frame, Bx′ denotes a decoding target block, B1′ denotes analready decoded block, B0′ denotes an undecoded block, BLu′, BLs′, BLh′,and Blm′ denote block boundaries of the decoding target block Bx′ at theupper, lower, left, and right sides, respectively, and P0′˜P9′ denotereference pixels corresponding to a decoding target pixel Px′. In thiscase, the arrangement of the reference pixels P0′˜P9′ with respect tothe decoding target pixel Px′ is identical to that described for thecoding process shown in FIG. 13(a), FIG. 13(b), and FIG. 14.

In the combination decoding method, initially, an image signalcorresponding to a single frame screen FG′ is divided correspondingly toplural blocks B′ constituting the single frame screen as shown in FIG.16(a) and FIG. 16(b), and a decoding process for the divided imagesignal is performed block by block.

This block-by-block decoding process is completed by performing ahorizontal process in which pixel values of pixels are successivelydecoded from the left to the right along each horizontal pixel line in ablock B′, for all of the horizontal pixel lines in the block from theuppermost line to the lower most line.

In this decoding process, as shown in FIG. 17, the pixel value of thedecoding target pixel Px′ is adaptively predicted from the pixel valuesof the reference pixels P0′˜P9′ positioned in the vicinity of thedecoding target pixel, and code words used for the decoding process ofthe decoding target pixel Px′ are adaptively selected according to apredicted value obtained by the prediction.

However, the combination coding method in which the block-by-blockcoding process and the pixel-by-pixel coding process are combined hasthe following drawbacks.

In this combination coding method, since the coding process proceedsblock by block, when the coding target pixel Px abuts the right boundaryBLm of the coding target block Bx as shown in FIG. 15, the referencepixels P2, P6, and P7 corresponding to the coding target pixel Px areuncoded pixels.

In this case, when the pixel value of the coding target pixel Px ispredicted with reference to the pixel values of the uncoded pixels P2,P6, and P7 and the pixel value of the coding target pixel Px is codedusing the predicted value, a coded image signal corresponding to thiscoding target pixel Px cannot be accurately decoded on the decodingside. More specifically, in order to accurately decode a coded imagesignal which is obtained by coding the pixel value of the coding targetpixel Px using the predicted value, the predicted value of the decodingtarget pixel Px′ used in the decoding process must agree with thepredicted value of the coding target pixel Px corresponding to thedecoding target pixel Px′, used in the coding process. In other words,on the coding side, the reference pixel values referred to forgenerating the predicted value of the coding target pixel Px mustcompletely agree with the reference pixel values referred to forgenerating the predicted value of the decoding target pixel Px′corresponding to the coding target pixel Px.

For this reason, for example, as shown in FIG. 15, when the codingprocess for the coding target pixel Px is performed, if the predictedvalue of the coding target pixel Px is generated with reference to thepixel values of the uncoded pixels P2, P6, and P7 among the referencepixels P0˜P9 corresponding to the decoding target pixel Px, as shown inFIG. 18, when the decoding process for the decoding target pixel Px′ isperformed on the decoding side, the predicted value of the decodingtarget pixel Px′ is generated with reference to the pixel values of thereference pixels P0′˜P9′ corresponding to the decoding target pixel Px′,but among the reference pixels P0′˜P9′, pixel values of the undecodedpixels P2′, P6′, and P7′ are not obtained on the decoding side, so thatthe pixel value of the decoding target pixel Px′ corresponding to thecoding target pixel Px cannot be decoded.

Therefore, in the conventional combination coding method, in order toavoid the problem that the decoding process becomes difficult whenuncoded pixels are included in the reference pixels P0˜P9 correspondingto the coding target pixel Px as described above, there is taken acountermeasure in which the predicted value of the coding target pixelPx is generated with the pixel values of the uncoded pixels beingregarded as a fixed value which has been previously set (e.g., 0), andthe coding process for the coding target pixel Px is performed usingthis predicted value.

Although the combination coding method with the above-mentionedcountermeasure enables the decoding process to be accurately performedfor all of the pixels in each block using their predicted values, sincethe pixel values of the reference pixels being uncoded pixels areuniformly replaced with a fixed value, the correlation of pixel valuesbetween the uncoded pixel and the coded pixel is degraded, resulting inthe problem that the efficiency in predicting the coding target pixel,i.e., the accuracy of the predicted value of the coding target pixel, isdegraded.

The present invention is subjected to solving the above-describedproblems, and has an object to provide an image processing apparatus andan image processing method, which can combine an adaptive pixel-by-pixelcoding process and a block-by-block coding process, without degradingcorrelation of pixel values between uncoded pixels and coded pixels,with avoiding that decoding of a coded image signal becomes difficult,and to provide a data recording medium in which an image processingprogram for realizing the image processing method is recorded.

It is another object of the present invention to provide an imageprocessing apparatus and an image processing method, which can performan accurate decoding process for a coded image signal which has beencoded, without degrading an efficiency in predicting coding targetpixels, and to provide a data recording medium in which an imageprocessing program for realizing the image processing method isrecorded.

DISCLOSURE OF THE INVENTION

An image processing apparatus according to the present invention (claim1) being an image coding apparatus for successively coding pixel valuesconstituting an image signal on the basis of pixel values of pluralperipheral pixels positioned in the vicinity of a coding target pixel,comprises blocking means for blocking an image signal comprising pluralpixel values corresponding to a single image display region into blockseach comprising a prescribed number of pixels, and outputting, block byblock, the prescribed number of pixel values constituting the imagesignal in each block; pixel value replacing means for replacing a pixelvalue of an uncoded pixel among the peripheral pixels with a pseudopixel value that is obtained from a pixel value of a coded pixel amongthe peripheral pixels on the basis of a prescribed rule; and codingmeans for receiving the image signal comprising plural pixel valuescorresponding to each block, performing, for each block, a codingprocess in which the respective pixel values are successively coded onthe basis of the pixel value of the coded pixel and the pseudo pixelvalue of the uncoded pixel, and outputting an coded image signal.

Since the image coding apparatus thus constructed is provided with thepixel value replacing means for replacing a pixel value of an uncodedpixel among plural peripheral pixels corresponding to a coding targetpixel with a pseudo pixel value that is obtained on the basis of a pixelvalue of a coded pixel among the plural peripheral pixels, when ablock-by-block coding process for an image signal corresponding to asingle image display region is performed pixel by pixel, with referenceto the pixel values of the peripheral pixels in the vicinity of thecoding target pixel, even though the coding target pixel abuts on theblock boundary and the plural reference peripheral pixels include anuncoded pixel, a pseudo pixel value having an improved correlation withthe pixel values of the other peripheral pixels can be referred to as apixel value of the uncoded pixel. Since this pseudo pixel value isobtained from a pixel value of a coded pixel in the vicinity of thecoding target pixel, on the decoder side, with respect to an undecodedpixel to be referred to when a decoding target pixel is decoded, apseudo pixel value obtained from a pixel value of a decoded pixel can bereferred to in place of its pixel value.

Therefore, it is possible to combine an adaptive pixel-by-pixel codingprocess and a block-by-block coding process without degradingcorrelation of pixel values between the uncoded pixel and the codingtarget pixel, with avoiding that decoding of a coded signal becomesdifficult on the decoding side.

Thereby, influence of transmission error can be converged block byblock, and the coding efficiency can be improved as compared with thesimple block-by-block coding process, and further, a decoding processfor a coded signal which has been coded without degrading the predictionefficiency of the coding target pixel can be performed accurately.

According to the present invention (claim 2), in the image codingapparatus of claim 1, the pixel value replacing means is constructed soas to employ, as a pseudo pixel value of the uncoded pixel, a pixelvalue of a coded pixel which is positioned at the shortest spatialdistance from the uncoded pixel.

Since, in the image coding apparatus thus constructed, the pixel valueof the coded pixel which is the nearest to the uncoded pixel is employedas the pseudo pixel value of the uncoded pixel, the correlation betweenthe pseudo pixel value of the uncoded pixel and the pixel values of theperipheral pixels can be improved.

According to the present invention (claim 3), in the image codingapparatus of claim 1, the pixel value replacing means is constructed soas to employ, as a pseudo pixel value of the uncoded pixel, a pixelvalue of a coded pixel which is positioned at the shortest spatialdistance from the uncoded pixel and on the same horizontal scanning lineas the uncoded pixel.

Since, in the image coding apparatus thus constructed, the pixel valueof the coded pixel which is the nearest to the uncoded pixel and on thesame horizontal scanning line as the uncoded pixel is employed as thepseudo pixel value of the uncoded pixel, the pseudo pixel value of theuncoded pixel can be obtained by a simple method, such as to hold thepixel value of the coded pixel for a prescribed period of time.

According to the present invention (claim 4), in the image codingapparatus of claim 1, the coding means is constructed so as to comprisea prediction value generator for generating a prediction pixel value forthe coding target pixel, on the basis of the pixel value of the codedpixel and the pseudo pixel value of the uncoded pixel; and an encoderfor coding a difference value between the pixel value of the codingtarget pixel and the prediction pixel value of the coding target pixel,and outputting the coded difference value, block by block, as a codedimage signal.

Since, in the image coding apparatus thus constructed, the differencevalue between the pixel value of the coding target pixel and theprediction pixel value is coded, the coding efficiency can be increasedby reducing the code quantity required for coding the pixel value of thecoding target pixel.

According to the present invention (claim 5), in the image codingapparatus of claim 1 or 4, the coding means is constructed so as toselect code words for coding the pixel value of the coding target pixel,on the basis of the pixel value of the coded pixel and the pseudo pixelvalue of the uncoded pixel.

Since, in the image coding apparatus thus constructed, code words forcoding the pixel value of the coding target pixel are selected on thebasis of the pixel value of the coded pixel and the pseudo pixel valueof the uncoded pixel, in a coding process for an image signal, a highlyefficient coding process can be realized by adaptively changing the codewords pixel by pixel.

According to the present invention (claim 6), in the image codingapparatus of claim 1 or 4, the coding means is constructed so as toselect a probability table corresponding to codes for arithmeticallycoding the pixel value of the coding target pixel, on the basis of thepixel value of the coded pixel and the pseudo pixel value of the uncodedpixel, and to perform an arithmetic coding process for the coding targetpixel on the basis of the selected probability table.

Since, in the image coding apparatus thus constructed, a probabilitytable corresponding to arithmetic codes for coding the coding targetpixel is selected on the basis of the pixel value of the coded pixel andthe prediction pixel value of the uncoded pixel, and coding of the pixelvalue for the coding target pixel is performed, a highly efficientcoding process can be performed by adaptively changing the probabilitytable, pixel by pixel, in the arithmetic coding process.

According to the present invention (claim 7), the image coding apparatusof claim 1 further comprises local decoding means for decoding the pixelvalue of the coded pixel to generate a local decoded pixel value;wherein the pixel value replacing means is constructed so as to replacethe pixel value of the uncoded pixel with a pseudo pixel value that isobtained from a local decoded pixel value corresponding to the codedpixel on the basis of a prescribed rule; and the coding means isconstructed so as to perform a non-reversible coding process for thepixel value of the coding target pixel, on the basis of the localdecoded pixel value of the coded pixel and the pseudo pixel value of theuncoded pixel.

Since, in the image processing apparatus thus constructed, thenon-reversible coding process for the pixel value of the coding targetpixel is performed on the basis of the decoded pixel value obtained bydecoding the pixel value of the coded pixel, the pixel values of theperipheral pixels to be referred to in the non-reversible coding processfor the coding target pixel can be identical to pixel values ofperipheral pixels to be referred to in a decoding process for a decodingtarget pixel, whereby a coded image signal obtained by thenon-reversible coding on the basis of the pixel values of the peripheralpixels can be accurately decoded on the decoder side.

According to the present invention (claim 8), in the image codingapparatus of claim 7, the coding means is constructed so as to comprisea prediction value generator for generating a prediction pixel value forthe coding target pixel, on the basis of the local decoded pixel valueof the coded pixel and the pseudo pixel value of the uncoded pixel; andan encoder for coding a difference value between the pixel value of thecoding target pixel and the prediction pixel value of the coding targetpixel, and outputting the coded difference value, block by block, as acoded image signal.

Since, in the image coding apparatus thus constructed, the predictionpixel value for the coding target pixel is generated from the pixelvalue of the coded pixel and the pseudo pixel value of the uncodedpixel, and the difference value between the pixel value of the codingtarget pixel and its prediction pixel value is coded, the codingefficiency can be improved by reducing the code quantity required forcoding the pixel value of the coding target pixel.

According to the present invention (claim 9), in the image codingapparatus of claim 7 or 8, the coding means is constructed so as toselect code words for coding the pixel value of the coding target pixel,on the basis of the local decoded pixel value of the coded pixel and thepseudo pixel value of the uncoded pixel.

Since, in the image coding apparatus thus constructed, code words forcoding the pixel value of the coding target pixel are selected on thebasis of the pixel value of the coded pixel and the pseudo pixel valueof the uncoded pixel, in a coding process for an image signal, a highlyefficient coding process can be realized by adaptively changing the codewords pixel by pixel.

According to the present invention (claim 10), in the image codingapparatus of claim 7 or 8, the coding means is constructed so as toselect a probability table corresponding to codes for arithmeticallycoding the pixel value of the coding target pixel, on the basis of thepixel value of the coded pixel and the pseudo pixel value of the uncodedpixel, and to perform an arithmetic coding process for the coding targetpixel on the basis of the selected probability table.

Since, in the image coding apparatus thus constructed, a probabilitytable corresponding to arithmetic codes for coding the coding targetpixel is selected on the basis of the pixel value of the cod ed pixeland the pseudo pixel value of the uncoded pixel, and coding of the pixelvalue for the coding target pixel is perform ed, a highly efficientcoding process can be performed by adaptively changing the probabilitytable, pixel by pixel, in the arithmetic coding process.

An image decoding apparatus according to the present invention (claim11) being an image decoding apparatus for decoding, block by block, acoded image signal which is obtained by performing, for each blockcomprising a prescribed number of pixels, a process in which pixelvalues constituting an image signal are successively coded on the basisof pixel values of plural peripheral pixels positioned in the vicinityof a coding target pixel, comprises pixel value replacing means forreplacing a pixel value of an undecoded pixel among plural peripheralpixels positioned in the vicinity of a decoding target pixel, with apseudo pixel value that is obtained from a pixel value of a decodedpixel among the plural peripheral pixels on the basis of a prescribedrule; decoding means for receiving the coded image signal comprisingplural pixel values corresponding to each block, performing, block byblock, a decoding process in which the respective pixel values aresuccessively decoded on the basis of the pixel value of the decodedpixel and the pseudo pixel value of the undecoded pixel, and outputtinga decoded image signal corresponding to each block; and inverse blockingmeans for combining the decoded image signals corresponding to therespective blocks to convert these signals to a decoded image signalhaving a scanning line structure; wherein the decoded image signalhaving a scanning line structure is output as a reproduced image signalcorresponding to a single image display screen.

Since the image decoding apparatus thus constructed is provided with thepixel value replacing means for replacing a pixel value of an undecodedpixel among plural peripheral pixels positioned in the vicinity of adecoding target pixel, with a pseudo pixel value that is obtained on thebasis of a pixel value of a decoded pixel among the plural peripheralpixels, when a block-by-block decoding process for a coded image signalcorresponding to a single image display region is performed pixel bypixel, with reference to the pixel values of the peripheral pixels inthe vicinity of the decoding target pixel, even though the decodingtarget pixel abuts on the block boundary and the plural referenceperipheral pixels include an undecoded pixel, a pseudo pixel valuehaving an improved correlation with the pixel values of the otherperipheral pixels can be referred to as a pixel value of the undecodedpixel.

Therefore, it is possible to realize a decoding method in which anadaptive pixel-by-pixel decoding process and a block-by-block decodingprocess are combined without degrading correlation of pixel valuesbetween the undecoded pixel and the decoding target pixel. Thereby, itis possible to accurately decode a coded image signal that has beencoded by a coding method in which an adaptive pixel-by-pixel codingprocess and a block-by-block coding process are combined.

According to the present invention (claim 12), in the image decodingapparatus of claim 11, the pixel value replacing means is constructed soas to employ, as a pseudo pixel value of the undecoded pixel, a pixelvalue of a decoded pixel which is positioned at the shortest spatialdistance from the undecoded pixel.

Since, in the image decoding apparatus thus constructed, the pixel valueof the decoded pixel which is the nearest to the undecoded pixel isemployed as the pseudo pixel value of the undecoded pixel, thecorrelation between the pseudo pixel value of the undecoded pixel andthe pixel values of the peripheral pixels can be improved.

According to the present invention (claim 13), in the image decodingapparatus of claim 11, the pixel value replacing means is constructed soas to employ, as a pseudo pixel value of the undecoded pixel, a pixelvalue of a decoded pixel which is positioned at the shortest spatialdistance from the undecoded pixel and on the same horizontal scanningline as the undecoded pixel.

Since, in the image decoding apparatus thus constructed, the pixel valueof the decoded pixel which is the nearest to the undecoded pixel and onthe same horizontal scanning line as the undecoded pixel is employed asthe pseudo pixel value of the undecoded pixel, the pseudo pixel value ofthe undecoded pixel can be obtained by a simple method, such as to holdthe pixel value of the decoded pixel for a prescribed period of time.

According to the present invention (claim 14), in the image decodingapparatus of claim 11, the decoding means is constructed so as tocomprise a prediction value generator for generating a prediction pixelvalue for the decoding target pixel, on the basis of the pixel value ofthe decoded pixel and the pseudo pixel value of the undecoded pixel; anda decoder for generating a decoded pixel value by decoding the pixelvalue of the decoding target pixel, and adding the prediction pixelvalue of the decoding target value to the decoded pixel value.

Since, in the image decoding apparatus thus constructed, the predictionpixel value of the decoding target pixel is generated from the pixelvalue of the decoded pixel and the pseudo pixel value of the undecodedpixel, and the prediction pixel value is added to the decoded pixelvalue of the decoding target pixel, it is possible to accurately decodea coded image signal that is obtained by coding a difference valuebetween a pixel value of a coding target pixel and its prediction pixelvalue.

According to the present invention (claim 15), in the image decodingapparatus of claim 11 or 14, the decoding means is constructed so as toselect code words for decoding the pixel value of the decoding targetpixel, on the basis of the pixel value of the decoded pixel and thepseudo pixel value of the undecoded pixel.

Since, in the image decoding apparatus thus constructed, code words fordecoding the pixel value of the decoding target pixel are selected onthe basis of the pixel value of the decoded pixel and the pseudo pixelvalue of the undecoded pixel, it is possible to accurately decode acoded image signal that is obtained in a highly efficient coding processin which the code words are adaptively changed pixel by pixel.

According to the present invention (claim 16), in the image decodingapparatus of claim 11 or 14, the decoding means is constructed so as toselect a probability table corresponding to codes for arithmeticallydecoding the pixel value of the decoding target pixel, on the basis ofthe pixel value of the decoded pixel and the pseudo pixel value of theundecoded pixel, and to perform an arithmetic decoding process for thedecoding target pixel on the basis of the selected probability table.

Since, in the image decoding apparatus thus constructed, a probabilitytable corresponding to arithmetic codes for decoding the decoding targetpixel is selected on the basis of the pixel value of the decoded pixeland the pseudo pixel value of the undecoded pixel, and decoding of thepixel value for the decoding target pixel is performed, it is possibleto accurately decode a coded image signal that is obtained in anarithmetic coding process in which the probability table is adaptivelychanged pixel by pixel.

An image coding method according to the present invention (claim 17) isan image coding method for performing, for each block comprising aprescribed number of pixels, a coding process in which pixel valuesconstituting an image signal are successively coded on the basis ofpixel values of plural peripheral pixels positioned in the vicinity of acoding target pixel, wherein with respect to a coded pixel among theplural peripheral pixels, its pixel value is set as a reference pixelvalue and, with respect to an uncoded pixel among the plural peripheralpixels, a pseudo pixel value, which is obtained from a pixel value of acoded pixel among the peripheral pixels on the basis of a prescribedrule, is set as a reference pixel value; and the pixel value of thecoding target pixel is coded on the basis of the reference pixel valuesset for the plural peripheral pixels for the coding target pixel,thereby generating a coded image signal corresponding to the imagesignal.

Since, in the image coding method thus constructed, the image signal isblocked correspondingly to respective blocks on a single image displayregion, and the pixel value of the coding target pixel in each block iscoded with reference to the pixel values of the peripheral pixels, andat this time, when the peripheral pixel is a coded pixel, its pixelvalue is referred to, and when the peripheral pixel is an uncoded pixel,a pseudo pixel value that is obtained from a pixel value of a codedpixel is referred to in place of its pixel value, in the coding process,even though the coding target pixel abuts on the block boundary and theplural reference peripheral pixels include an uncoded pixel, a pseudopixel value having an improved correlation with the pixel values of theother peripheral pixels can be referred to as a pixel value of theuncoded pixel. Since this pseudo pixel value is obtained from a pixelvalue of a coded pixel in the vicinity of the coding target value, onthe decoder side, with respect to an undecoded pixel to be referred towhen a decoding target pixel is decoded, a pseudo pixel value obtainedfrom a pixel value of a decoded pixel can be referred to in place of itspixel value.

Therefore, it is possible to combine an adaptive pixel-by-pixel codingprocess and a block-by-block coding process without degradingcorrelation of pixel values between the uncoded pixel and the codingtarget pixel, with avoiding that decoding of a coded signal becomesdifficult on the decoding side.

Thereby, influence of transmission error can be converged block byblock, and the coding efficiency can be improved as compared with thesimple block-by-block coding process, and further, a decoding processfor a coded signal without degrading the prediction efficiency of thecoding target pixel can be performed accurately.

An image decoding method according to the present invention (claim 18)is an image decoding method for decoding, block by block, a coded imagesignal which is obtained by performing, for each block comprising aprescribed number of pixels, a process in which pixel valuesconstituting an image signal are successively coded on the basis ofpixel values of plural peripheral pixels positioned in the vicinity of acoding target pixel, wherein with respect to a decoded pixel amongplural peripheral pixels, its pixel value is set as a reference pixelvalue and, with respect to an undecoded pixel among the pluralperipheral pixels, a pseudo pixel value, which is obtained from a pixelvalue of a decoded pixel among the peripheral pixels on the basis of aprescribed rule, is set as a reference pixel value; and a pixel value ofa decoding target pixel is decoded on the basis of the reference pixelvalues set for the plural peripheral pixels for the decoding targetpixel, thereby generating a decoded image signal corresponding to thecoded image signal.

Since, in the image decoding method thus constructed, a process in whichthe pixel value of the decoding target pixel in each block issuccessively decoded with reference to the pixel values of theperipheral pixels, is performed for each block comprising plural pixels,and at this time, when the peripheral pixel is a decoded pixel, itspixel value is referred to, and when the peripheral pixel is anundecoded pixel, a pseudo pixel value that is obtained from a pixelvalue of a decoded pixel is referred to in place of its pixel value, inthe decoding process, even though the decoding target pixel abuts on theblock boundary and the plural reference peripheral pixels include anundecoded pixel, a pseudo pixel value having an improved correlationwith the pixel values of the other peripheral pixels can be referred toas a pixel value of the undecoded pixel.

Therefore, it is possible to realize a decoding method in which anadaptive pixel-by-pixel decoding process and a block-by-block decodingprocess are combined without degrading correlation of pixel valuesbetween the undecoded pixel and the decoding target pixel. Thereby, itis possible to accurately decode a coded image signal that has beencoded by a coding method in which an adaptive pixel-by-pixel codingprocess and a block-by-block coding process are combined.

A data recording medium according to the present invention (claim 19) isa data recording medium containing a program for performing an imagesignal coding process or decoding process, wherein the program is animage processing program for making a computer perform processing of animage signal by the image coding method of claim 17 or the imagedecoding method of claim 18.

Employing this data recording medium, error propagation when atransmission error occurs can be converged block by block, and thecoding efficiency can be improved as compared with the simpleblock-by-block coding process, and further, it is possible to accuratelyperform a decoding process for a coded signal without degrading aprediction efficiency of a coding target pixel, and to accurately decodea coded image signal that has been coded by a coding method in which anadaptive pixel-by-pixel coding process and a block-by-block codingprocess are combined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a construction of an image codingapparatus according to a first embodiment of the present invention.

FIG. 2 is a flowchart showing process steps of generating a pixel valueof an uncoded pixel in a coding process by the image coding apparatus.

FIG. 3 is a block diagram illustrating a construction of an image codingapparatus according to a second embodiment of the present invention.

FIG. 4 is a block diagram illustrating a construction of an image codingapparatus according to a third embodiment of the present invention.

FIG. 5 is a block diagram illustrating a construction of an image codingapparatus according to a fourth embodiment of the present invention.

FIG. 6 is a block diagram illustrating a construction of an image codingapparatus according to a first modification of the fourth embodiment.

FIG. 7 is a block diagram illustrating a construction of an image codingapparatus according to a second modification of the fourth embodiment.

FIG. 8 is a block diagram illustrating a construction of an imagedecoding apparatus according to a fifth embodiment of the presentinvention.

FIG. 9 is a flowchart showing process steps of generating a pixel valueof an undecoded pixel in a decoding process by the image decodingapparatus.

FIG. 10 is a block diagram illustrating a construction of an imagedecoding apparatus according to a sixth embodiment of the presentinvention.

FIG. 11 is a block diagram illustrating a construction of an imagedecoding apparatus according to a seventh embodiment of the presentinvention.

FIG. 12(a), FIG. 12(b), and FIG. 12(c) are diagrams showing a datarecording medium containing a program for implementing the codingprocess by the image coding apparatus or the decoding process by theimage decoding apparatus according to any of the aforementionedembodiments in a computer system.

FIG. 13(a) and FIG. 13(b) are schematic diagrams showing division of asingle frame screen into plural blocks, in a block-by-block codingprocess.

FIG. 14 is a schematic diagram for explaining an adaptive pixel-by-pixelcoding process.

FIG. 15 is a schematic diagram for explaining problems in a combinationcoding method in which the block-by-block coding process and thepixel-by-pixel coding process are combined.

FIG. 16(a) and FIG. 16(b) are schematic diagram showing division of asingle frame screen into plural blocks, in a block-by-block decodingprocess.

FIG. 17 is a schematic diagram for explaining an adaptive pixel-by-pixeldecoding process.

FIG. 18 is a schematic diagram for explaining problems in a combinationdecoding method in which the block-by-block decoding process and thepixel-by-pixel decoding process are combined.

BEST EMBODIMENTS FOR EXECUTING THE INVENTION

A description is given of respective embodiments of the presentinvention using FIGS. 1 to 12.

Embodiment 1

FIG. 1 is a block diagram illustrating a construction of an image codingapparatus 101 according to a first embodiment of the present invention.

This image coding apparatus 101 comprises a blocking unit 2 for dividingan input image signal Is correspondingly to plural blocks constituting asingle image display region (one frame), an encoder 16 a for receivingan output Bs from the blocking unit 2 and performing reversible codingfor a pixel value of a coding target pixel to be coded in a codingtarget block to be coded, with reference to a prediction pixel value(hereinafter referred to simply as a prediction value) Sp of the codingtarget pixel, and a prediction value generation unit 110 for generatingthe prediction pixel value Sp.

This prediction value generation unit 110 includes a mass storage mainmemory 4 capable of storing pixel values of pixels constituting theinput image signal by, for example, numbers corresponding to one frame,and first and second auxiliary memories 6 a and 6 b of different holdperiods of time, for temporarily holding pixel values M output from themain memory 4. In this case, the main memory 4 has a construction inwhich the pixel values corresponding to the reference pixels P0˜P9 forthe coding target pixel Px to be processed by the encoder 16 a (see FIG.14) are successively output from the stored pixel values, during aperiod of time required for a coding process of a pixel value. The firstauxiliary memory 6 a has a construction in which the pixel values Msuccessively output from the main memory 4 are delayed by one pixel, andthe second auxiliary memory 6 b has a construction in which the pixelvalues M successively output from the main memory 4 are delayed by twopixels.

Further, the prediction value generation unit 110 includes a counter 8for receiving the input image signal Is per frame and counting thenumber of pixel values, and a coded/uncoded decision unit 10 fordeciding whether the pixel value being output from the main memory 4 isa pixel value of a coded pixel which has already been coded by theencoder 16 a or a pixel value of an uncoded pixel which has not beencoded yet, according to an output Cout from the counter 8 andinformation BNs relating to vertical and horizontal block numbers ineach frame, supplied from the outside. In this case, the decision unit10 also performs a process of measuring a distance between an uncodedpixel and a coded pixel that is on the same horizontal scanning line asthe uncoded pixel and nearest to the uncoded pixel, with the number ofpixels. The counter 8 is reset when the pixel values of all pixelsconstituting one frame have been input.

Furthermore, the prediction value generation unit 110 includes aselector switch 12 for selecting and outputting one of the outputs M,Ma, and Mb from the main memory 4, and the first and second auxiliarymemories 6 a and 6 b, respectively, according to an output from thecoded/uncoded decision unit 10, and a prediction value generator 14 forreceiving outputs Sout from the selector switch 12 as the pixel valuesof the reference pixels P0˜P9 required for generating a prediction pixelvalue for the coding target pixel Px, and generating the predictionpixel value Sp for the coding target pixel Px.

In the image coding apparatus 101, the encoder 16 a performs a codingprocess for a difference value between the pixel value of the codingtarget pixel Px output from the blocking unit 2 and its prediction pixelvalue, and outputs a coded difference value as a coded pixel valuecorresponding to the coding target pixel Px. In this coding process,code words for coding the pixel value of the coding target pixel Px areselected on the basis of the prediction pixel value obtained from thepixel values of the reference pixels P0˜P9.

A description is given of the operation.

FIG. 2 is a flowchart showing the coding process of the image codingapparatus 101, and the flow of the coding process will be describedbriefly along this flowchart.

When an image signal is input to the image coding apparatus 101, pixelvalues constituting the image signal are successively stored in the mainmemory 4, and the main memory 4 outputs pixel values of plural referencepixels (peripheral pixels) P0˜P9 located in the vicinity of a codingtarget pixel Px, which reference pixels are referred to when the codingtarget pixel Px is coded (step S1).

Next, the first reference pixel in the plural reference pixels obtainedby the main memory 4 is regarded as a decision target pixel (step S2),and the coded/uncoded decision unit 10 decides whether the decisiontarget pixel is a coded pixel or not (step S3). Since in thiscoded/uncoded decision unit 10 the increment of the counter output Coutis synchronized with the operation of successively outputting the pixelvalues of the ten reference pixels P0˜P9 from the main memory 4according to a reference clock, the position of the reference pixelbeing output from the main memory 4, relative to the coding target pixelPx, can be detected by the counter output Cout.

As the result of the decision, when the decision target reference pixelis a coded pixel, its pixel value is received as a reference pixel valueby the prediction value generator 14 (step S5), and when the decisiontarget reference pixel is not a coded pixel, a reference pixel value(pseudo pixel value) of this reference pixel is generated from itsperipheral coded pixels, and received by the prediction value generator14 (step S4).

Next, it is decided whether all of the reference pixel values requiredfor coding the coding target pixel are received by the prediction valuegenerator 14 (step S6), and when all of the reference pixel valuesrequired are not received, the next reference pixel in the referencepixels obtained by the main memory 4 is regarded as the decision target(step S8), and the processes in steps S3˜S6 are repeated. On the otherhand, when all of the reference pixel values required are received bythe prediction value generator 14, a prediction pixel value for thecoding target pixel Px is generated according to the reference pixelvalues by the prediction value generator 14 (step S7).

Thereafter, the pixel value of the coding target pixel Px is received bythe encoder 16 a and, simultaneously, the prediction pixel value of thecoding target pixel Px is received from the prediction value generator14 (step S9), and in the encoder 16 a, a coding process for the pixelvalue of the coding target pixel Px is carried out using the predictionpixel value (step S10).

A detailed description is now given of the operation of the image codingapparatus 101 in the coding process for the image signal, and thespecific operations of the respective units in this apparatus in stepsS1˜S10.

When an input image signal Is is input to the image coding apparatus101, plural pixels constituting the input image signal Is are groupedcorrespondingly to blocks constituting one frame, each block comprisingplural pixels, by the blocking unit 2, and the pixel valuescorresponding to the pixels in each block are sent to the encoder 16 aand, in the encoder 16 a, a coding process in which pixel values ofcoding target pixels Px are coded pixel by pixel with reference toreference pixel values, is carried out block by block.

At this time, the pixel values constituting the input image signal Is ofscanning line structure are successively stored in the main memory 4,and pixel values of reference pixels P0˜P9 corresponding to therespective coding target pixels Px are output at a fixed read cycle(step S1). An output M from the main memory 4 is temporarily held in thefirst and second auxiliary memories 6 a and 6 b. The first auxiliarymemory 6 a holds the output from the main memory 4 for a periodcorresponding to one read cycle, and the second auxiliary memory 6 bholds it for a period corresponding to two read cycles.

In the counter 8, with the first pixel value in one frame as a reference, the number of input pixel values is counted in accordance with theinput image signal Is, and this count value Cout is output toward thecoded/uncoded decision unit 10. In the coded/uncoded decision unit 10,information BNs relating to vertical and horizontal block numbers in oneframe is input from the outside, and a reference pixel designated by theblock number information BNs and the output from the counter 8 isregarded as a decision target pixel to be decided whether it is a codedpixel or an uncoded pixel. For example, when a coding target pixel Px isdecided as shown in FIG. 14, reference pixels P0˜P9 for this codingtarget pixel are decided, and the pixel P0 whose pixel value is outputfrom the main memory 4 first is regarded as a decision target pixel(step S2).

At this time, the pixel value of the reference pixel P0 is held in thefirst and second auxiliary memories 6 a and 6 b for one read period, andtwo read periods, respectively. In the coded/uncoded decision unit 10,the position of the coding target pixel in the coding target block iscalculated on the basis of the counter output Cout and the block numberinformation BNs, and it is decided whether the reference pixel is acoded pixel or an uncoded pixel (step S3) and, according to the resultof the decision, the selector switch 12 is controlled.

Since the reference pixel P0 is a coded pixel as shown in FIG. 14, asthe result of the decision by the coded/uncoded decision unit 10 (stepS3), the selector switch 12 is controlled by the coded/uncoded decisionunit 10 to select the output M from the main memory 4, whereby the pixelvalue of the reference pixel P0 is stored, as a reference pixel value,in the prediction value generator 14.

Thereafter, in the coded/uncoded decision unit 10, it is decided whetheror not the pixel values of all the reference pixels for the codingtarget pixel Px are received by the prediction value generator 14 (stepS6). In this case, since the reference pixel values of all the referencepixels P0˜P9 are not received by the prediction value generator 14, thecoded/uncoded unit 10 regards the pixel value of the reference pixel P1,which is output from the main memory 4 after the pixel value of thereference pixel P0, as a pixel value of a decision target pixel (stepS8). Since the reference pixel P1 is a coded pixel like the referencepixel P0, the pixel value of the reference pixel P1 is subjected to theprocesses in steps S3, S5, S6, and S8.

Subsequently, the coded/uncoded unit 10 regards the pixel value of thereference pixel P2, which is output from the main memory 4 after thepixel value of the reference pixel P1, as a pixel value of a decisiontarget pixel (step S8). Since this reference pixel P2 is an uncodedpixel adjacent to the coded pixel, unlike the reference pixels P0 andP1, the selector switch 12 is controlled by the coded/uncoded decisionunit 10 to select the output Ma from the first auxiliary memory 6 a,whereby the pixel value of the reference pixel P1 is stored in theprediction value generator 14 as a pseudo pixel value of the referencepixel P2 (step S4). Thereafter, the processes in step S6 and step S8 areexecuted.

Further, the reference pixels P3˜P5 are subjected to the processes insteps S3, S5, S6, and S8 in the same manner as described for thereference pixel P0, and the reference pixel P6 is subjected to theprocesses in steps S3, S4, S6, and S8 in the same manner as describedfor the reference pixel P2.

Subsequently, the coded/uncoded unit 10 regards the pixel value of thereference pixel P7, which is output from the main memory 4 after thepixel value of the reference pixel P6, as a pixel value of a decisiontarget pixel (step S8). Since this reference pixel P7 is an uncodedpixel that is positioned across one pixel from the coding target pixelPx, unlike the reference pixels P0˜P6, the selector switch 12 iscontrolled by the coded/uncoded decision unit 10 to select the output Mbfrom the second auxiliary memory 6 b, whereby the pixel value of thereference pixel P5 is stored in the prediction value generator 14 as apseudo pixel value of the reference pixel P7 (step S4). Thereafter, theprocesses in step S6 and step S8 are executed.

Further, the reference pixels P8 and P9 are subjected to the processesin steps S3, S5, and S6 in the same manner as described for thereference pixel P0. At this time, the coded/uncoded unit 10 decides thatthe reference pixel values of all the reference pixels P0˜P9 arereceived by the prediction value generator 14, and the prediction valuegenerator 14 calculates a prediction pixel value Sp for the codingtarget pixel Px on the basis of the reference pixel values received(step S7).

Thereafter, the pixel value of the coding target pixel Px and theprediction pixel value Sp are received by the encoder 16 a (step S9),and a difference value between the pixel value of the coding targetpixel Px and its prediction pixel value Sp is subjected to a codingprocess, and the coded difference value is output as a coded signal ofthe coding target pixel Px (step S10). This coding process employs codewords selected on the basis of the prediction pixel value Sp.

In this way, the pixel values in the respective blocks in one frame aresuccessively coded. Since the first pixel in one frame has no codedpixel for reference in its vicinity, its coding process is carried outwith a prediction pixel value of 0.

As described above, according to the first embodiment of the invention,when a pixel value of an uncoded pixel is referred to, since a pixelvalue of a coded pixel in its vicinity is referred to, it is possible tocombine the adaptive pixel-by-pixel coding process and theblock-by-block coding process without degrading correlation of pixelvalues between the uncoded pixel and the coding target pixel, and withavoiding that decoding of a coded signal becomes difficult. Thereby,influence of transmission error can be converged block by block, and thecoding efficiency can be improved as compared with the simpleblock-by-block coding process, and further, on the decoding side, it ispossible to accurately perform a decoding process for a coded signalwhich has been coded with no degradation in prediction efficiency of thecoding target pixel.

More specifically, when a coding process for a coding target pixel hasbeen carried out with reference to pixel values of its peripheral pixelson the coding side, a decoding process for a decoding target pixel mustbe also carried out with reference to pixel values of its peripheralpixels on the decoding side, and further, the pixel values referred toin the coding process must agree with the pixel values referred to inthe decoding process.

In the conventional coding method, when the reference pixel is anuncoded pixel, a fixed value is used as its pixel value. In this case,however, there is a problem that the correlation of pixel values isdegraded.

On the other hand, according to the present invention, when thereference pixel is an uncoded pixel, a pseudo pixel value for theuncoded pixel is generated from coded reference pixels, according to aprescribed rule. For example, for the uncoded pixels P2, P6, and P7shown in FIG. 15, their pixel values are uniquely decided from the pixelvalues of the coded pixels P0, P1, P3, P4, P5, P8, and P9. The easiestmanner is to use a pixel value of a coded pixel positioned on the samehorizontal scanning line as and nearest to the uncoded pixel, as thepseudo pixel value for the uncoded pixel. In this case, the pixel valueof the uncoded pixel P2 is replaced with the pixel value of the codedpixel P1, and the pixel values of the uncoded pixels P6 and P7 arereplaced with the pixel value of the coded pixel P5.

Thus, the correlation of pixel values between the coded pixel and theuncoded pixel at the block boundary is increased, whereby a highlyefficient coding process can be realized using the same predictionmethod as used for the inside of block where the inter-pixel correlationis great.

Therefore, an image signal can be recorded and transmitted with less bitnumber, without deteriorating the image quality.

While in the first embodiment two auxiliary memories 6 a and 6 b ofdifferent hold times for holding the output M from the main memory 4 areprovided, only one auxiliary memory for temporarily holding the outputfrom the main memory 4 may be provided, and the time for holding thepixel value output from the main memory 4 may be changed by thecoded/uncoded decision unit 10 according to the distance between theuncoded pixel and the coding target pixel.

Further, while in the first embodiment only the prediction pixel valueof the coding target pixel Px is referred to in the coding process forthe image signal, the coding process may be performed on the basis of,not only the prediction pixel value of the coding target pixel Px, butalso the prediction probability that shows the accuracy of theprediction pixel value.

In this case, when the image signal being a target of the coding processis a binary shape signal, since the prediction pixel value is either “0”or “1”, only the prediction probability may be referred to, with theprediction pixel value being fixed to either “0” or “1”.

Hereinafter, as a second embodiment of the invention, an image codingapparatus for performing a coding process of a binary shape signal withreference to only a prediction probability in place of the predictionpixel value will be described, and as a third embodiment of theinvention, an image coding apparatus for performing a coding process ofa multivalued image signal with reference to both the prediction pixelvalue and a prediction probability will be described.

Embodiment 2

FIG. 3 is a block diagram illustrating a construction of an image codingapparatus 102 according to a second embodiment of the present invention.In the figure, the same reference numerals as those shown in FIG. 1designate the same parts as in the image coding apparatus 101 describedin the first embodiment.

This image coding apparatus 102 includes a prediction probabilitygenerator 22 in place of the prediction value generator 14 in the imagecoding apparatus 101 according to the first embodiment, and has such aconstruction as to perform a coding process for a binary shape signalwhose pixel value is either “0” or “1”.

The prediction probability generator 22 has a construction in which as aprediction probability, a probability of a pixel value of a codingtarget pixel Px matching with a prediction pixel value is obtained frompixel values of reference pixels P0˜P9 for the coding target pixel Px,and then a prediction probability signal Sk for the coding target pixelPx is output toward the encoder 16 b.

In this case, since this image coding apparatus 102 is for a codingprocess of a binary shape signal, in the encoder 16 b, a predictionvalue which is subtracted from the pixel value of the coding targetpixel Px is set at either “0” or “1”. The encoder 16 b has aconstruction in which, when the prediction probability is high, sincethe probability of the pixel value of the coding target pixel Pxmatching with its prediction pixel value is high, a difference valuebetween the pixel value of the coding target pixel Px and the predictionpixel value is coded by a coding method which increases the codingefficiency when the difference value is 0, and on the other hand, whenthe prediction probability is low, since the probability of the pixelvalue of the coding target pixel Px matching with the prediction pixelvalue is low, a difference value between the pixel value of the codingtarget pixel Px and the prediction pixel value is coded by a codingmethod which increases the coding efficiency when the difference valueis not 0.

A description is given of the function and effect.

Here, the operation identical to that of the image coding apparatusaccording to the first embodiment is not described.

Also in the binary shape signal coding process by the image codingapparatus 102 thus constructed, with respect to uncoded pixels among thereference pixels P0˜P9 for the coding target pixel Px, their pixelvalues are generated from the pixel values of the coded pixels as in theimage coding apparatus 101 according to the first embodiment, and thepixel values corresponding to all the reference pixels P0˜P9 are storedin the prediction probability generator 22.

In the prediction probability generator 22, on the basis of the pixelvalues of the reference pixels P0˜P9, a prediction probability for thecoding target pixel Px is obtained. This prediction probabilityinformation Sk is output from the prediction probability generator 22 tothe encoder 16 b, and in the encoder 16 b, a difference value betweenthe pixel value of the coding target pixel Px and a previously setprediction pixel value is subjected to a coding process according to theprediction probability information Sk.

As described above, according to the second embodiment of the invention,in the image coding apparatus 102 for performing a coding process of abinary shape signal, a coding target pixel adjacent to the blockboundary, for which generation of a prediction value is difficult, canbe coded while suppressing degradation in coding efficiency. Therefore,influence of transmission error can be converged block by block, and thecoding efficiency can be increased as compared with the simpleblock-by-block coding process, and further, on the decoding side, it ispossible to accurately perform a decoding process for a coded signalwithout degrading the efficiency in predicting the coding target pixel.

By the way, a coding process based on a prediction probability isdisclosed in, for example, International Standard JBIG (Joint Bi-levelImage Coding Experts Group), but a coding method disclosed in thisStandard is to perform a pixel-by-pixel coding process (notblock-by-block), so that reference pixels are always coded pixels, andit does not disclose the problem when the reference pixels are uncodedpixels in the block-by-block coding process, which problem is to besolved by the present invention, and a countermeasure as to how to setpixel values corresponding to these uncoded pixels.

Embodiment 3

FIG. 4 is a block diagram illustrating a construction of an image codingapparatus 103 according to a third embodiment of the present invention.In the figure, the same reference numerals as those shown in FIG. 1designate the same parts as in the image coding apparatus 101 describedin the first embodiment.

This image coding apparatus 103 is for performing a coding process for amultivalued image signal as described above, and has a construction inwhich a prediction probability generation unit 130 includes a predictionprobability generator 22 in addition to the prediction value generator14 in the image coding apparatus 101 described in the first embodiment.

Like the prediction value generator 14, the prediction probabilitygenerator 22 has a construction to receive outputs Sout from theselector switch 12, to successively store pixel values of referencepixels P0˜P9 for a coding target pixel Px, and to output, on the basisof these pixel values, a prediction probability Sk that shows theaccuracy of a prediction pixel value Sp which is predicted on the basisof the reference pixels P0˜P9 in the prediction value generator 14.

The encoder 16 c has a construction in which the pixel value of thecoding target pixel is coded on the basis of the prediction pixel valueSp from the prediction value generator 14 and the prediction probabilitySk from the prediction probability generator 22.

To be specific, when the accuracy of the prediction pixel value is high,since the difference between the pixel value of the coding target pixeland the prediction pixel value is small, the encoder 16 c performs acoding process for the pixel value of the coding target pixel Px by acoding method which increases the coding efficiency when the differencebetween the pixel value of the coding target pixel and the predictionpixel value is small. On the other hand, when the accuracy of theprediction value is low, since the difference between the pixel value ofthe coding target pixel and the prediction pixel value is large,theencoder 16 c performs a coding process for the pixel value of the codingtarget pixel Px by a coding method which increases the coding efficiencywhen the difference between the pixel value and the prediction pixelvalue is relatively large.

Also in the multivalued image signal coding process by the image codingapparatus 103 thus constructed, with respect to uncoded pixels among thereference pixels P0˜P9 for the coding target pixel Px, their pixelvalues are generated from the pixel values of the coded pixels as in theimage coding apparatus 101 according to the first embodiment, and thepixel values corresponding to all the reference pixels P0˜P9 are storedin the prediction probability generator 22 and the prediction valuegenerator 14.

In the prediction value generator 14, a prediction pixel value of thecoding target pixel Px is obtained from the reference pixels P0˜P9 as inthe first embodiment. Further, in the prediction probability generator22, a prediction probability Sk for the coding target pixel Px isobtained on the basis of the pixel values of the reference pixels P0˜P9.

When the prediction pixel value Sp and the prediction probability Sk areoutput to the encoder 16 c, the encoder 16 c performs a coding processaccording to the prediction probability Sk for a difference valuebetween the pixel value of the coding target pixel Px and the predictionpixel value from the prediction value generator 14.

As described above, according to the third embodiment of the invention,in the image coding apparatus 103 for coding a multivalued signal, acoding target pixel adjacent to the block boundary, for which generationof a prediction value is difficult, can be coded while suppressingdegradation in coding efficiency. Therefore, influence of transmissionerror can be converged block by block, and the coding efficiency can beincreased as compared with the simple block-by-block coding process, andfurther, on the decoding side, it is possible to accurately perform adecoding process for a coded signal without degrading the efficiency inpredicting the coding target pixel.

Embodiment 4

FIG. 5 is a block diagram illustrating a construction of an image codingapparatus 104 according to a fourth embodiment of the present invention.In the figure, the same reference numerals as those shown in FIG. 1designate the same parts as in the image coding apparatus described inthe first embodiment.

This image coding apparatus 104 is different from the image codingapparatus 101 according to the first embodiment in that a blocked imagesignal Bs is subjected to a non-reversible coding process.

More specifically, the image coding apparatus 104 includes an encoder16d for performing a non-reversible coding process including a DCT(Discrete Cosine Transformation) process to an output Bs from theblocking unit 2, on the basis of a prediction pixel value from theprediction value generator 14, in place of the encoder 16 a forperforming a reversible coding process according to the firstembodiment. Further, a prediction value generation unit 140 in thisimage coding apparatus 104 includes a local decoder 24 that decodes anoutput Cs from the encoder 16d on the basis of the prediction pixelvalue Sp from the prediction value generator 14, wherein an output LDsfrom the local decoder 24 is stored in the main memory 4 as a pixelvalue of a decoded pixel, and the output LDs is input to the counter 8.The other construction is identical to that of the image codingapparatus 101 according to the first embodiment.

In the image coding apparatus 104 so constructed, when an input imagesignal Is is coded on the basis of a prediction pixel value Sp of acoding target pixel Px, in the prediction value generator 140 thatgenerates the prediction pixel value Sp, the local decoder 24 decodes anoutput Cs from the encoder 16 d with reference to the prediction pixelvalue Sp, and the decoded pixel value is stored in the main memory 4.

Therefore, in the image coding apparatus 104 performing a non-reversiblecoding process, the decoded pixel value is used for generating theprediction pixel value, whereby the coded image signal coded by thisimage coding apparatus can be decoded accurately in an image decodingapparatus.

Although in the fourth embodiment only the prediction pixel value of thecoding target pixel Px is referred to in the image signal non-reversiblecoding process, not only the prediction pixel value of the coding targetpixel Px but also the prediction probability showing the accuracy of theprediction pixel value may be referred to.

In this case, when the image signal subjected to the non-reversiblecoding process is a binary shape signal, since the prediction pixelvalue is either “0” or “1”, only the prediction probability may bereferred to, while fixing the prediction pixel value to either “0” or“1”.

Hereinafter, a description is given of an image coding apparatusaccording to a first modification of the fourth embodiment of theinvention, which performs a non-reversible coding process for a binaryshape signal with reference to only the prediction probability in placeof the prediction pixel value, and an image coding apparatus accordingto a second modification of the fourth embodiment, which performs anon-reversible coding process for a multivalued image signal withreference to both the prediction pixel value and the predictionprobability.

FIG. 6 is a block diagram illustrating a construction of an image codingapparatus 104 a according to the first modification of the fourthembodiment of the invention. In the figure, the same reference numeralsas those shown in FIG. 5 designate the same parts as in the image codingapparatus 104 according to the fourth embodiment.

This image coding apparatus 104 a includes a prediction probabilitygenerator 22 in place of the prediction value generator 14 in the imagecoding apparatus 104 according to the fourth embodiment, and performs acoding process for a binary shape signal whose pixel value is either “0”or “1”.

The prediction probability generator 22 has a construction in which as aprediction probability, a probability of a pixel value of a codingtarget pixel Px matching with a prediction pixel value is obtained frompixel values of reference pixels P0˜P9 for the coding target pixel Px,and then the prediction probability Sk for the coding target pixel Px isoutput toward the encoder 16 e and the local decoder 24.

In this case, since this image coding apparatus is for treating a binaryshape signal as its processing target, in the encoder 16 e, theprediction pixel value for the coding target pixel Px is set at either“0” or “1”. Like the encoder 16 b according to the second embodiment,this encoder 16 e has a construction in which, when the predictionprobability is high, a difference value between the pixel value of thecoding target pixel Px and the prediction pixel value is coded by acoding method which increases the coding efficiency when the differencevalue is 0, and on the other hand, when the prediction probability islow, a difference value between the pixel value of the coding targetpixel Px and the prediction pixel value is coded by a coding methodwhich increases the coding efficiency when the difference value is not0.

Further, the local decoder 24 performs a decoding process whileswitching the decoding method on the basis of the predictionprobability, like the encoder 16 e.

Here, the operation identical to that of the image coding apparatusaccording to the fourth embodiment is not described.

Also in the binary shape signal coding process by the image codingapparatus 104 a thus constructed, with respect to uncoded pixels amongthe reference pixels P0˜P9 for the coding target pixel Px, their pixelvalues are generated from the pixel values of the coded pixels as in theimage coding apparatus 104 according to the fourth embodiment, and thepixel values corresponding to all the reference pixels P0˜P9 are storedin the prediction probability generator 22.

In the prediction probability generator 22, on the basis of the pixelvalues of the reference pixels P0˜P9, a prediction probability Sk forthe coding target pixel Px is obtained. This prediction probability Skis output from the prediction probability generator 22 to the encoder 16eand the local decoder 24, and in the encoder 16 e, a non-reversiblecoding process according to the prediction probability Sk is performedfor a difference value between the pixel value of the coding targetpixel Px and a previously set prediction pixel value. At this time, thelocal decoder 24 performs a decoding process for an output Cs from theencoder 16 e according to the prediction probability Sk, whereby thepixel value of the coding target pixel Px is reproduced. This reproducedpixel value of the coding target pixel Px is stored in the main memory4.

As described above, according to the first modification of the fourthembodiment, in the image coding apparatus 104 a performing anon-reversible coding process for a binary shape signal, a coding targetpixel adjacent to the block boundary, for which generation of aprediction value is difficult, can be coded while suppressingdegradation in coding efficiency. Therefore, influence of transmissionerror can be converged block by block, and the coding efficiency can beincreased as compared with the simple block-by-block coding process, andfurther, on the decoding side, it is possible to accurately perform adecoding process for a coded signal without degrading the efficiency inpredicting the coding target pixel.

FIG. 7 is a block diagram illustrating a construction of an image codingapparatus 104 b according to the second modification of the fourthembodiment of the invention. In the figure, the same reference numeralsas those shown in FIG. 4 designate the same parts as in the image codingapparatus 104 according to the fourth embodiment.

This image coding apparatus 104 b is for performing a non-reversiblecoding process for a multivalued image signal as described above, andhas a construction in which a prediction probability generation unit 140b includes a prediction probability generator 22 in addition to theprediction value generator 14 in the image coding apparatus 104according to the fourth embodiment.

Like the prediction value generator 14, the prediction probabilitygenerator 22 has a construction to receive outputs Sout from theselector switch 12, to successively store pixel values of referencepixels P0˜P9 for a coding target pixel Px, and to output, on the basisof these pixel values, a prediction probability Sk that shows theaccuracy of a prediction pixel value Sp which is predicted on the basisof the reference pixels P0˜P9 in the prediction value generator 14.

The encoder 16f has a construction to perform a non-reversible codingprocess for the pixel value of the coding target pixel Px on the basisof the prediction pixel value Sp from the prediction value generator 14and the prediction probability Sk from the prediction probabilitygenerator 22, and the specific structure thereof is identical to that ofthe encoder 16 c according to the third embodiment.

Further, the local decoder 24 performs a decoding process whileswitching the coding method according to the prediction probability,like the encoder 16 e.

Also in the multivalued image signal coding process by the image codingapparatus 104 b thus constructed, with respect to uncoded pixels amongthe reference pixels P0˜P9 for the coding target pixel Px, their pixelvalues are generated from the pixel values of the coded pixels as in theimage coding apparatus 104 according to the fourth embodiment, and thepixel values corresponding to all the reference pixels P0˜P9 are storedin the prediction probability generator 22 and the prediction valuegenerator 14.

In the prediction value generator 14, a prediction pixel value of thecoding target pixel Px is obtained from the reference pixels P0˜P9 as inthe fourth embodiment. Further, in the prediction probability generator22, a prediction probability Sk for the coding target pixel Px isobtained on the basis of the pixel values of the reference pixels P0˜P9.

The prediction pixel value Sp and the prediction probability Sk areoutput to the encoder 16 f and the local decoder 24, and in the encoder16 f, a difference value between the pixel value of the coding targetpixel Px and a previously set prediction pixel value is subjected to acoding process according to the prediction probability Sk. At this time,the local decoder 24 performs a decoding process for an output Cs fromthe encoder 16 f according to the prediction probability Sk, whereby thepixel value of the coding target pixel Px is reproduced. This reproducedpixel value of the coding target pixel Px is stored in the main memory4.

Therefore, degradation in coding efficiency is avoided with respect toan image signal for which generation of a prediction value is difficult,and a significant increase in coding efficiency can be realized withrespect to an image signal for which a prediction value is easygenerated.

As described above, according to the second modification of the fourthembodiment, in the image coding apparatus 104 b performingnon-reversible coding of a multivalued signal, a coding target pixeladjacent to the block boundary, for which generation of a predictionvalue is difficult, can be coded while suppressing degradation in codingefficiency. Therefore, influence of transmission error can be convergedblock by block, and the coding efficiency can be increased as comparedwith the simple block-by-block coding process, and further, on thedecoding side, it is possible to accurately perform a decoding processfor a coded signal without degrading the efficiency in predicting thecoding target pixel.

Embodiment 5

FIG. 8 is a block diagram illustrating a construction of an imagedecoding apparatus 105 according to a fifth embodiment of the presentinvention.

The image decoding apparatus 105 according to this fifth embodimentreversibly decodes a coded image signal which has been reversibly codedby the image coding apparatus 101 according to the first embodiment.

This image decoding apparatus 105 includes a decoder 26 a for decodingan input coded image signal Cs for each of plural blocks constituting asingle image display region, more specifically, for decoding a pixelvalue of a decoding target pixel Px′ being a target of a decodingprocess, i.e., a coded signal which is obtained by coding a pixel valueof a coding target pixel Px, with reference to a prediction pixel valueof the decoding target pixel, an inverse blocking unit 30 for combiningdecoded image signals Ds corresponding to the respective blocks outputfrom the decoder 26 a to generate a reproduced image signal Rs having aprescribed scanning line structure, and a prediction value generationunit 150 for generating the prediction pixel value on the basis of pixelvalues of reference pixels P0′˜P9′ located in the vicinity of thedecoding target pixel Px′.

The prediction value generation unit 150 is similar to the predictionvalue generation unit 110 included in the image coding apparatus 101according to the first embodiment.

To be specific, the generation value generation unit 150 includes a massstorage main memory 4 capable of storing pixel values by, for example,numbers corresponding to one frame, and first and second auxiliarymemories 6 a and 6 b of different hold periods of time, for temporarilyholding pixel values M output from the main memory 4. In this case, themain memory 4 has a construction in which the pixel values correspondingto the reference pixels P0′˜P9′ for the decoding target pixel Px′ to beprocessed by the decoder 26 a (see FIG. 17) are successively output fromthe stored pixel values, during a period of time required for a codingprocess of a pixel value. The first auxiliary memory 6 a has aconstruction in which the pixel values M successively output from themain memory 4 are delayed by one pixel, and the second auxiliary memory6 b has a construction in which the pixel values M successively outputfrom the main memory 4 are delayed by two pixels.

Further, the prediction value generation unit 150 includes a counter 8for receiving the decoded image signal Ds per frame and counting thenumber of pixel values, and a decoded/undecoded decision unit 20 fordeciding whether the pixel value being output from the main memory 4 isa pixel value of a decoded pixel which has already been decoded by thedecoder 26 a or a pixel value of an undecoded pixel which has not beendecoded yet, according to an output Cout from the counter 8 andinformation BNs relating to vertical and horizontal block numbers ineach frame, supplied from the outside. In this case, the decision unit20 also performs a process of measuring a distance between an undecodedpixel and a decoded pixel that is on the same horizontal scanning lineas and nearest to the undecoded pixel, with the number of pixels. Thecounter 8 is reset when the pixel values of all pixels constituting oneframe are input.

Furthermore, the prediction value generation unit 150 includes aselector switch 12 for selecting and outputting one of the outputs M,Ma, and Mb from the main memory 4, and the first and second auxiliarymemories 6 a and 6 b, respectively, according to an output Scont fromthe decoded/undecoded decision unit 20, and a prediction value generator14 for receiving outputs Sout from the selector switch 12 as the pixelvalues of the reference pixels P0′˜P9′ required for generating aprediction pixel value for the decoding target pixel Px′, and generatingthe prediction pixel value Sp for the decoding target pixel Px′.

In the image decoding apparatus 105, the decoder 26 a decodes a codeddifference value for the coding target pixel Px which is input as acoded image signal from the outside to generate a decoded differencevalue, and adds the prediction pixel value Sp from the prediction valuegeneration unit 150 to the decoded difference value to generate adecoded pixel value of the decoding target pixel and output the same tothe inverse blocking unit 30.

A description is given of the operation.

FIG. 9 is a flowchart showing the decoding process of the image decodingapparatus 105, and the flow of the decoding process will be describedbriefly along this flowchart.

When a coded image signal Cs is input to this image coding apparatus105, the decoder 26 a performs a decoding process for the coded imagesignal Cs, according to a prediction signal Sp from the prediction valuegeneration unit 150.

At this time, a plurality of decoded pixel values output from thedecoder 26 a and corresponding to one frame are successively stored inthe main memory 4, and the main memory 4 outputs pixel values of pluralreference pixels (peripheral pixels) P0′˜P9′ located in the vicinity ofa decoding target pixel Px′, which reference pixels are referred to whenthe decoding target pixel Px′ is decoded (step S11).

Next, the first reference pixel among the plural reference pixelsobtained by the main memory 4 is regarded as a decision target pixel(step S12), and the decoded/undecoded decision unit 20 decides whetherthe decision target pixel is a decoded pixel or not (step S13). Since inthis decoded/undecoded decision unit 20 the increment of the counteroutput Cout is synchronized with the operation of successivelyoutputting the pixel values of the ten reference pixels P0′˜P9′ from themain memory 4 according to a reference clock, the position of thereference pixel being output from the main memory 4, relative to thedecoding target pixel Px′, can be detected by the counter output Cout.

As the result of the decision, when the decision target reference pixelis a decoded pixel, its pixel value is received as a reference pixelvalue by the prediction value generator 14 (step S15), and when thedecision target reference pixel is not a decoded pixel, a referencepixel value (pseudo pixel value) of this reference pixel is generatedfrom its peripheral decoded pixels, and received by the prediction valuegenerator 14 (step S14).

Next, it is decided whether all of the reference pixel values requiredfor decoding the decoding target pixel are received by the predictionvalue generator 14 (step S16), and when all of the reference pixelvalues required are not received, the next reference pixel in thereference pixels obtained by the main memory 4 is regarded as thedecision target (step S18), and the processes in steps S13˜S16 arerepeated. On the other hand, when all of the reference pixel valuesrequired are received by the prediction value generator 14, a predictionpixel value for the decoding target pixel Px′ is generated according tothe reference pixel values by the prediction value generator 14 (stepS17).

Thereafter, the pixel value of the decoding target pixel Px′ is receivedby the decoder 26 a and, simultaneously, the prediction pixel value ofthe decoding target pixel Px′ is received from the prediction valuegenerator 14 (step S19), and in the decoder 26 a, a decoding process forthe pixel value of the decoding target pixel Px′ is carried out usingthe prediction pixel value (step S20).

A detailed description is now given of the operation of the imagedecoding apparatus 105 in the decoding process for the image signal, andthe specific operations of the respective units in this apparatus insteps S11˜S20.

When a coded image signal Cs is input to the image decoding apparatus105, the coded image signal Cs is transmitted to the decoder 26 a, andin the decoder 26 a, a decoding process in which pixel values ofdecoding target pixels Px′ are decoded pixel by pixel with reference toreference pixel values, is performed block by block.

At this time, the pixel values output from the decoder 26 a andconstituting one frame are successively stored in the main memory 4, andpixel values of reference pixels P0′˜P9′ for each decoding target pixelPx′ are output at a fixed read cycle (step S11). An output M from themain memory 4 is temporarily held in the first and second auxiliarymemories 6 a and 6 b. The first auxiliary memory 6 a holds the outputfrom the main memory 4 for a period corresponding to one read cycle, andthe second auxiliary memory 6 b holds it for a period corresponding totwo read cycles.

In the counter 8, with the first pixel value in one frame as areference, the number of input pixel values is counted in accordancewith outputs Ds from the decoder 26 a, and this count value Cout isoutput toward the decoded/undecoded decision unit 20. In thisdecoded/undecoded decision unit 20, information BNs relating to verticaland horizontal block numbers in one frame is input from the outside, anda reference pixel designated by the block number information BNs and theoutput from the counter 8 is regarded as a decision target pixel to bedecided whether it is a decoded pixel or an undecoded pixel. Forexample, when a decoding target pixel Px′ is decided as shown in FIG.17, reference pixels P0′˜P9′ for this decoding target pixel are decided,and the pixel P0′ whose pixel value is output from the main memory 4first is regarded as a decision target pixel (step S12).

At this time, the pixel value of the reference pixel P0′ is held in thefirst and second auxiliary memories 6 a and 6 b for one read period, andtwo read periods, respectively. In the decoded/undecoded decision unit20, the position of the decoding target pixel in the decoding targetblock is calculated on the basis of the counter output Cout and theblock number information BNs, it is decided whether the reference pixelis a decoded pixel or an undecoded pixel, and according to the result ofthe decision, the selector switch 12 is controlled.

Since this pixel P0′ is a decoded pixel as shown in FIG. 17, as theresult of the decision by the decoded/undecoded decision unit 20 (stepS13), the selector switch 12 is controlled by the decoded/undecodeddecision unit 20 to select the output M from the main memory 4, wherebythe pixel value of the reference pixel P0′ is stored, as a referencepixel value, in the prediction value generator 14.

Thereafter, in the decoded/undecoded decision unit 20, it is decidedwhether or not the pixel values of all the reference pixels for thedecoding target pixel Px′ are received by the prediction value generator14 (step S16). In this case, since the reference pixel values of all thereference pixels P0′˜P9′ are not received by the prediction valuegenerator 14, the decoded/undecoded decision unit 20 regards the pixelvalue of the reference pixel P1′, which is output from the main memory 4after the pixel value of the reference pixel P0′, as a pixel value of adecision target pixel (step S18). Since the reference pixel P1′ is adecoded pixel like the reference pixel P0′, the pixel value of thereference pixel P1′ is subjected to the processes in steps S13, S15,S16, and S18.

Subsequently, the decoded/undecoded decision unit 20 regards the pixelvalue of the reference pixel P2′, which is output from the main memory 4after the pixel value of the reference pixel P1′, as a pixel value of adecision target pixel (step S18). Since this reference pixel P2′ is anundecoded pixel adjacent to the decoded pixel, unlike the referencepixels P0′ and P1′, the selector switch 12 is controlled by thedecoded/undecoded decision unit 20 to select the output Ma from thefirst auxiliary memory 6 a, whereby the pixel value of the referencepixel P1′ is stored in the prediction value generator 14 as a pseudopixel value of the reference pixel P2′ (step S14). Thereafter, theprocesses in step S16 and step S18 are executed.

Further, the reference pixels P3′˜P5′ are subjected to the processes insteps S13, S15, S16, and S18 like the reference pixel P0′, and thereference pixel P6′ is subjected to the processes in steps S13, S14,S16, and S18 like the reference pixel P2′.

Subsequently, the decoded/undecoded decision unit 20 regards the pixelvalue of the reference pixel P7′, which is output from the main memory 4after the pixel value of the reference pixel P6′, as a pixel value of adecision target pixel (step S18). Since this reference pixel P7′ is anundecoded pixel that is positioned across one pixel from the decodingtarget pixel, unlike the reference pixels P0′˜P6′, the selector switch12 is controlled by the decoded/undecoded decision unit 20 to select theoutput Mb from the second auxiliary memory 6 b, whereby the pixel valueof the reference pixel P5′ is stored in the prediction value generator14 as a pseudo pixel value of the reference pixel P7′ (step S14).Thereafter, the processes in step S16 and step S18 are executed.

Further, the reference pixels P8′ and P9′ are subjected to the processesin steps S13, S15, and S16 like the reference pixel P0′. At this time,the decoded/undecoded decision unit 20 decides that the reference pixelvalues of all the reference pixels P0′˜P9′are received by the predictionvalue generator 14, and the prediction value generator 14 calculates aprediction pixel value for the decoding target pixel Px′ on the basis ofthe reference pixel values received (step S17).

Thereafter, the pixel value of the decoding target pixel Px′ and theprediction pixel value are received by the decoder 26 a (step S19), anda value obtained by adding the pixel value of the decoding target pixelPx′ and the prediction pixel value is output as a decoded image signalDs of the decoding target pixel Px′ (step S20).

In this way, the pixel values in the respective blocks in one frame aresuccessively decoded. Since the first pixel in one frame has no decodedpixel for reference, its decoding process is carried out with aprediction pixel value of 0.

In the inverse blocking unit 30 decoded image signals Ds are combined soas to correspond to a single frame screen to output a reproduced imagesignal Rs of a scanning line structure.

As described above, according to the fifth embodiment of the invention,when a pixel value of an undecoded pixel is referred to, since a pixelvalue of a coded pixel in its vicinity is referred to, the adaptivepixel-by-pixel decoding process and the block-by-block decoding processcan be performed without degrading correlation of pixel values betweenthe undecoded pixel and the decoding target pixel. Thereby, it ispossible to accurately decode a coded image signal Cs which has beenprocessed in a coding method in which influence of transmission errorcan be converged block by block and the coding efficiency can beimproved as compared with the simple block-by-block coding process.

While in the fifth embodiment two auxiliary memories 6 a and 6 b ofdifferent times for holding the output M from the main memory 4 areprovided, only one auxiliary memory for temporarily holding the outputfrom the main memory 4 may be provided, and the time for holding thepixel value output from the main memory 4 may be changed by thedecoded/undecoded decision unit 20 according to the distance between theundecoded pixel and the decoding target pixel.

Further, while in the fifth embodiment only the prediction pixel valueof the decoding target pixel Px′ is referred to in the decoding processfor the image signal, the decoding process may be performed on the basisof, not only the prediction pixel value of the decoding target pixelPx′, but also the prediction probability that shows the accuracy of theprediction pixel value.

In this case, when the image signal being a target of the decodingprocess is a binary shape signal, since the prediction pixel value iseither “0” or “1”, only the prediction probability may be referred to,with the prediction pixel value being fixed to either “0” or “1”.

Hereinafter, as a sixth embodiment of the invention, an image decodingapparatus for performing a decoding process of a binary shape signalwith reference to only a prediction probability in place of theprediction pixel value will be described, and as a seventh embodiment ofthe invention, an image decoding apparatus for performing a decodingprocess of a multivalued image signal with reference to both theprediction pixel value and a prediction probability will be described.

Embodiment 6

FIG. 10 is a block diagram illustrating a construction of an imagedecoding apparatus according to a sixth embodiment of the presentinvention. In the figure, the same reference numerals as those shown inFIG. 8 designate the same parts as in the image decoding apparatus 105according to the fifth embodiment.

The image decoding apparatus 106 according to the sixth embodimentincludes a prediction probability generator 22 in place of theprediction value generator 14 in the image decoding apparatus 105according to the fifth embodiment, and performs a decoding process for abinary shape signal whose pixel value is either “0” or “1”.

The prediction probability generator 22 has a construction in which as aprediction probability Sk, a probability of a pixel value of a decodingtarget pixel Px′ matching with its prediction pixel value is obtainedfrom pixel values of reference pixels P0′˜P9′ for the decoding targetpixel Px′, and then the prediction probability Sk for the decodingtarget pixel Px′ is output toward the decoder 26 b.

The decoder 26 b has a construction to perform a decoding process for acoded image signal Cs which has been coded by the image coding apparatus102 according to the second embodiment.

A description is given of the function and effect.

Here, the operation identical to that of the image coding apparatusaccording to the first embodiment is not described.

Also in the binary shape signal decoding process by the image decodingapparatus 105 thus constructed, with respect to undecoded pixels amongthe reference pixels P0′˜P9′ for the decoding target pixel Px′, theirpixel values are generated from the pixel values of the decoded pixelsas in the image decoding apparatus 105 according to the fifthembodiment, and the pixel values corresponding to all the referencepixels P0′˜P9′ are stored in the prediction probability generator 22.

In the prediction probability generator 22, on the basis of the pixelvalues of the reference pixels P0′˜P9′, a prediction probability Sk forthe decoding target pixel Px′ is obtained. This prediction probabilitySk is output from the prediction probability generator 22 to the decoder26 b, and in the decoder 26 b, a difference value between the pixelvalue of the decoding target pixel Px′ and a previously set predictionpixel value is subjected to a decoding process according to theprediction probability Sk.

As described above, according to the sixth embodiment of the invention,in the image decoding apparatus 106 for decoding a coded image signalobtained by coding a binary image signal, it is possible to realize adecoding process corresponding to a coding process capable of coding acoding target pixel adjacent to the block boundary, for which generationof a prediction value is difficult, while suppressing degradation incoding efficiency.

Embodiment 7

FIG. 11 is a block diagram illustrating a construction of an imagedecoding apparatus according to a seventh embodiment of the invention.In the figure, the same reference numerals as those shown in FIG. 8designate the same parts as in the image coding apparatus 105 accordingto the fifth embodiment.

The image decoding apparatus 107 according to the seventh embodiment isfor performing a coding process for a multivalued image signal asdescribed above, and has a construction to perform a decoding processfor a coded image signal Cs which has been coded by the image codingapparatus 103 according to the third embodiment.

More specifically, a prediction probability generation unit 170 in theimage decoding apparatus 107 includes a prediction probability generator22 in addition to the prediction value generator 14 in the image codingapparatus 105 according to the fifth embodiment.

Like the prediction value generator 14, the prediction probabilitygenerator 22 has a construction to receive outputs Sout from theselector switch 12, to successively store pixel values of referencepixels P0′˜P9′ for a decoding target pixel Px′, and to output, on thebasis of these pixel values, a prediction probability Sk that shows theaccuracy of a prediction pixel value Sp which is predicted on the basisof the reference pixels P0′˜P9′ in the prediction value generator 14.

The decoder 26 c performs a decoding process corresponding to the codingprocess by the encoder 16 c according to the third embodiment, and thedecoder 26 c has a construction in which the pixel value of the decodingtarget pixel Px′ is decoded on the basis of the prediction pixel signalSp from the prediction value generator 14 and the prediction probabilitySk from the prediction probability generator 22.

A description is given of the function and effect.

Here, the operation identical to that of the image coding apparatusaccording to the fifth embodiment is not described.

Also in the multivalued image signal decoding process by the imagedecoding apparatus 107 thus constructed, with respect to undecodedpixels among the reference pixels P0′˜P9′ for the decoding target pixelPx′, their pixel values are generated from the pixel values of thedecoded pixels as in the image decoding apparatus 105 according to thefifth embodiment, and the pixel values corresponding to all thereference pixels P0′˜P9′ are stored in the prediction probabilitygenerator 22 and the prediction value generator 14.

In the prediction value generator 14, a prediction pixel value of thedecoding target pixel Px′ is obtained from the reference pixels P0′˜P9′as in the fifth embodiment. Further, in the prediction probabilitygenerator 22, a prediction probability for the decoding target pixel Px′is obtained on the basis of the pixel values of the reference pixelsP0′˜P9′.

When the prediction pixel value Sp and the prediction probability Sk areoutput to the decoder 26 c the decoder 26 c performs a decoding processaccording to the prediction probability Sk for a value obtained byadding the pixel value of the decoding target pixel Px′ and theprediction pixel value from the prediction value generator 14. The pixelvalue of the decoding target pixel Px′ so reproduced is stored in themain memory 4.

As described above, according to the seventh embodiment of theinvention, in the image decoding apparatus 107 for decoding a codedimage signal obtained by coding a multivalued image signal, it ispossible to realize a decoding process corresponding to a coding processcapable of coding a coding target pixel adjacent to the block boundary,for which generation of a prediction value is difficult, whilesuppressing degradation in coding efficiency.

Although in the fifth, sixth and seventh embodiments, described areimage decoding apparatuses for decoding coded image signals that havebeen reversibly coded by the image coding apparatuses 101, 102 and 103according to the first, second and third embodiments, respectively, byconstructing the decoder 26 a so as to perform a decoding processcorresponding to non-reversible coding, the image decoding apparatuses105, 106 and 107 according to the fifth, sixth and seventh embodimentscan correspond to the image coding apparatuses 104, 104 a and 104 baccording to the fourth embodiment, and the first and secondmodifications thereof, respectively.

Further, by recording a coding of decoding program for implementing theconstruction of the coding apparatus or decoding apparatus according toany of the aforementioned embodiments on a data recording medium such asa floppy disk, the process described in any of the embodiments can beexecuted easily in individual computer systems.

FIG. 12(a) is a diagram for explaining a case where a coding or decodingprocess according to any of the aforementioned embodiments is executedin a computer system using a floppy disk in which the coding or decodingprogram is stored.

FIG. 12(b) shows a front view of a floppy disk, its cross section, and afloppy disk, and FIG. 12(a) shows an example of a physical format of thefloppy disk as a recording medium body. The floppy disk FD is containedin a case F, plural tracks Tr are concentrically formed on the surfaceof the disk from the outer circumference toward the inner circumference,and each track is divided into 16 sectors Se in the angular direction.Therefore, in the floppy disk containing the above-mentioned program, ina region allocated on the floppy disk FD, data as the program isrecorded.

FIG. 12(c) shows a structure for performing recording/reproduction ofthe program on the floppy disk FD. When the program is recorded on thefloppy disk FD, data as the program from a computer system Csis iswritten via a floppy disk drive. When the above-mentioned coding ordecoding apparatus is constructed in the computer system by the programin the floppy disk, the program is read from the floppy disk by thefloppy disk drive and transmitted to the computer system.

Although in the above description a floppy disk is employed as a datarecording medium, an optical disk may be employed. Further, therecording medium is not limited to these disks, and anything may beemployed as long as a program, for example, an IC card or a ROMcassette, can be recorded therein.

In the second and third embodiments and the first and secondmodifications of the fourth embodiment, there is described an example inwhich the coding method is changed according to the predictionprobability, and in the fifth and sixth embodiments, there is describedan example in which the decoding method is changed according to theprediction probability, but code word (coding table) can be changedaccording to the prediction probability. Especially when coding isperformed with arithmetic code, by updating a probability tablecorresponding to the arithmetic code according to the predictionprobability, the image coding apparatuses according to the second andthird embodiments and the first and second modifications of the fourthembodiment, and the image decoding apparatuses according to the fifthand sixth embodiments can be realized by simple structures, and in thiscase, the effect on practical use is considerable.

In the present invention, when a prediction pixel value of a codingtarget pixel is predicted with reference to pixel values of pluralperipheral pixels located in its vicinity, for uncoded pixels among theperipheral pixels, pseudo pixel values are generated using pixel valuesof coded pixels among the peripheral pixels, but according to theposition of the coding target pixel in the block, as a group ofperipheral pixels to be referred to when generating its prediction pixelvalue, peripheral pixels in different arrangement in the vicinity of thecoding target pixel may be employed.

For example, in a specific description with reference to FIG. 14, whenthe coding target pixel Px is positioned at the boundary of the block,only the peripheral pixels P0, P1, P3, P4, P5, P8, and P9 are used asreference pixels, and when the coding target pixel Px is not positionedat the boundary of the block, all of the peripheral pixels P0, P1, P2,P3, P4, P5, P6, P7, P8, and P9 are used as reference pixels. When thecoding target pixel Px is coded, the code word is switched between codeword constructed by only the peripheral pixels P0, P1, P3, P4, P5, P8,and P9 and code word constructed by all of the peripheral pixels P0, P1,P2, P3, P4, P5, P6, P7, P8, and P9.

In other words, the encoder is constructed so that it has plural codewords corresponding to positions of the coding target pixel Px in theblock, and the code word is changed according to the position of thecoding target pixel Px.

Even in such a construction, it can be easily known from the embodimentsof the present invention that the same effects as provided by the imagecoding apparatuses are obtained.

Further, the decoder is also constructed so that it has plural codewords corresponding to positions of the decoding target pixel Px′ in theblock shown in FIG. 17, and the code word is changed according to theposition of the decoding target pixel Px′, whereby the same effects asprovided by the image decoding apparatuses of the aforementionedembodiments are obtained.

APPLICABILITY IN INDUSTRY

As described above, the image coding apparatuses, image coding methods,image decoding apparatuses, image decoding methods, and data recordingmedia according to the present invention can improve coding efficiencyin a compression process for image signals, and are very valuable forrealizing an image coding process and an image decoding process in asystem performing transmission or storage of image signals and,especially, are suitable for compression and decompression processes ofmoving pictures based on standards such as MPEG4.

What is claimed is:
 1. An image decoding apparatus for decoding, blockby block, each block having a square form and comprising N×N pixels, acoded image signal obtained by performing, for each block comprising aprescribed number of pixels, a process in which pixel values comprisingan image signal are successively coded on the basis of pixel values ofplural peripheral pixels positioned in the vicinity of a coding targetpixel, comprising: pixel value replacing means for replacing a pixelvalue of an undecoded pixel among plural peripheral pixels positioned inthe vicinity of a decoding target pixel, said peripheral pixelsincluding at least three lines of pixels and the target pixel is in thethird line, with a pseudo pixel value obtained from a pixel value of adecoded pixel among the plural peripheral pixels; wherein the pixelvalue replacing means employs, as the pseudo pixel value of theundecoded pixel, a pixel value of a decoded pixel which is positioned atthe shortest distance from the undecoded pixel and on the samehorizontal scanning line among said three lines of pixels; decodingmeans for (a) receiving the coded image signal comprising plural pixelvalues corresponding to each block, (b) performing, block by block, adecoding process in which the respective pixel values use the pixelvalues of at least one decoded pixel positioned in each of a previouslydecoded block, the decoding target block, and a pseudo pixel value of anundecoded pixel, and wherein the respective pixel values are decoded onthe basis of i) the pixel value of a decoded pixel positioned in apreviously decoded block for the peripheral pixel positioned in thepreviously decoded block; ii) the pixel value of a decoded pixelpositioned in the decoding target block for the peripheral pixelspositioned in the decoding target block; and iii) a pseudo pixel valueof an undecoded pixel for the peripheral pixels positioned in anundecoded block,  and (c) outputting a decoded image signalcorresponding to each block; and inverse blocking means for combiningdecoded image signals corresponding to the respective blocks.
 2. Animage decoding apparatus according to claim 1, wherein in said at leastthree lines of pixels the first line has at least three pixels, thesecond line has at least five pixels, and the third line has at leasttwo pixels.
 3. An image decoding apparatus as defined in claim 1 whereinthe decoding means selects code words for decoding the pixel value ofthe decoding target pixel, on the basis of the pixel value of thedecoded pixel and the pseudo pixel value of the undecoded pixel.
 4. Animage decoding apparatus as defined in claim 1 wherein the decodingmeans selects a probability table corresponding to codes forarithmetically decoding the pixel value of the decoding target pixel, onthe basis of the pixel value of the decoded pixel and the pseudo pixelvalue of the undecoded pixel, and performs an arithmetic decodingprocess for the decoding target pixel on the basis of the selectedprobability table.